Which of the following statements is true? a.)tt and tt plants have the same phenotype. b.)a person with type o blood has two recessive genes. c.)intelligence is controlled by a single gene. d.)in codominance, both genes are recessive.

The correct answer is option B, that is, a person with type O blood has two recessive genes.  

An illustration of a trait determined by a single gene is blood type. Each human exhibits two copies of the gene for blood type on chromosome pair number 9. One of the copy is inherited from the father and the other one is inherited from the mother. An individual's blood type is determined by which allele he or she inherits from the parents.  

In case of blood type O, O is a recessive allele. If an A or B is inherited along with the O gene, the A or B gene decides the blood type of an individual. An individual is considered to have type O only if that person inherits two O genes, that is, a person with type O blood exhibits two recessive genes.  

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

i think its white blood cells

i hope this